1. Field of the Invention
The present invention involves a method of assay of CD4+ T cells of a person infected with HIV to determine the immunodeficiency of the person. More specifically, the invention relates to a method of assay for the extent of depletion of subpopulations of CD4+ T cells which express specific .beta. chains of the T cell receptor. The invention also involves additional assays, products, and methods of vaccination related to the above.
2. Description of Background Art
The importance of T cell counts in HIV infected individuals has already been recognized. Currently, the National Institute of Allergies and Infectious Diseases (NIAID) has recommended that the CD4+ T cell counts in HIV-infected patients be determined two to four times per year. Healthy individuals normally have 800-1000 CD4+ cells/.mu.l of blood. NIAID recommends that AZT treatment begin when CD4 levels drops to 500 CD4+ cells/.mu.l and that when levels drop below 200 CD4+ cells/.mu.l that AZT be discontinued and pentamidine therapy be undertaken. CD4+ cell counts are currently performed using flow cytometry. FDA approval has recently been sought for a method of counting CD4+ cells by solubilizing CD4 receptors and quantifying by means of a microtiter calorimetric immunoassay.
The cause of T cell depletion in HIV infected individuals has remained a mystery, however. It is unlikely that T cell depletion is a direct result of HIV infection. Although HIV is known to infect CD4+ T cells by binding to the surface protein, only a fraction of CD4+ T cells, around 1 in 10000 are actually infected with the virus. A number of theories have been proposed to explain the depletion phenomenon but none have been proven or disproved.
One theory is that HIV requires the presence of another pathogen to deplete immune cell populations. The second pathogen may be opportunistic and depress the immune system simply by its presence. The second pathogen may also work as a "co-factor" working in concert with the HIV virus to directly suppress T cell populations. Mycoplasmas have been proposed as one candidate for co-factor pathogen status.
A second theory is that HIV triggers an autoimmune reaction that destroys immune cells. There is no agreement as to which of the possible markers on immune cells provides the binding site for the autoimmune antibodies. One group has proposed that antibodies to HIV envelope protein (gp120) could recognize and bind to the major histocompatibility (MHC) proteins. These anti-MHC antibodies would then cause the depletion of the T cells.
A third theory is that HIV-infected T cells fuse with uninfected cells to produce syncytia consisting of non-functioning cells. Thus, a single HIV-infected cell could bring about the demise of many non-infected T cells. The observation of syncytia formation was made in vitro, and no in vivo results have yet been published to substantiate this hypothesis.
Finally, some researchers have proposed that T cell depletion is caused by a HIV protein acting as a superantigen. Normally, when antigen is presented to a T cell, interaction of the alpha and beta chains of the T cell receptor with antigen is required for T cell activation. Superantigens interact only with the beta chain and are able to bypass the requirement of alpha chain interaction. By removing the alpha chain requirement, superantigens are able to activate a much larger proportion of the T cell subpopulation. Furthermore, T cells stimulated by superantigens eventually lose their ability to respond to antigens and undergo a programmed cell death known as apoptosis.
There is no agreement as to which HIV protein might have superantigen properties and, indeed, there has been no direct proof of superantigen involvement in T cell depletion. Some researchers have proposed that HIV gp120 may be the superantigen. Others have proposed that the nef gene produced by an open reading frame at the 3' end of the HIV genome may be the superantigen.
It is an object of the invention to delineate the pattern of T cell depletion in HIV infected individuals in order to develop effective intervention therapies to slow or halt T cell depletion and to more effectively time preventive treatment for opportunistic diseases.